1. Technical Field
The present disclosure relates to a bonded magnet, a component for the bonded magnet, and a production method of the bonded magnet.
2. Description of the Related Art
Permanent magnets have been used in various applications such as an electric motor and a loudspeaker. For example, a small electric motor includes a field coil that is arranged in the outer part, and a rotor that is arranged in the inner part. Permanent magnets are arranged on the surface of rotor (SPM) or buried inside the rotor (IPM). For example, on the conditions that SPM type small electric motors have the same numbers of magnetic poles and the same size of permanent magnets, in order to provide a higher torque, the magnetic flux density of the permanent magnet on the surface of the rotor is required to be higher. As such a high magnetic flux density permanent magnet, a sintered magnet containing an earth element has been used such as Nd2Fe14B.
Also, a bonded magnet has been used which includes magnet powder distributed in a plastic material. Such a bonded magnet can be formed by using dies in compression molding, injection molding, or the like. Accordingly, its dimensional accuracy can be easily increased as compared with the sintered magnet. In addition, the bonded magnet can be integrally formed with another component. Additionally, the bonded magnet is lightweight. For these reasons, the bonded magnet is used in various applications. However, the bonded magnet is essentially constructed of resin as a binder. The maximum volumetric ratio of magnet powder is approximately 70% which is added to the binder. Accordingly, its magnetic properties will be necessarily reduced by 30%. As a result, its magnetic force is smaller as compared with the sintered magnet. For example, its energy product is only approximately one third of a sintered magnet of NdFeB. Therefore, the bonded magnet has not been used for an electric motor that is primarily required to provide a high torque. From this viewpoint, a bonded magnet is needed which has a high magnetic flux density to be used for an electric motor, and the like.
Japanese Examined Publication No. JP H06-105,644 B; International Publication No. WO 2012/090,841 A; U.S. Pat. No. 5,019,796 B Specification; and Japanese Laid-Open Patent Publication No. JP H10-308,308 A disclose permanent magnets that are devised to increase their magnetic flux densities. These permanent magnets produces a single magnetic circuit which is used in the molding or magnetic orientation. That is, a closed magnetic circuit (closed circuit) is formed so that the magnetic flux of the magnet is curved or expanded in the magnetic field of the closed circuit. In the case of the process using the single magnetic circuit, it is difficult to control the magnetic orientation of a bonded magnet (to magnetize a bonded magnet). The reason is that when the magnetic field produced by the single magnetic circuit has a low strength part, even if the low strength part is small, the desired orientation of the low strength part will not be completely achieved. For this reason, in the case where such a bonded magnet including the incomplete orientation part is used as a cylindrical columnar bonded magnet for a rotor of an electric motor, its surface magnetic flux will be much lower as compared with the sintered magnet. Therefore, to use a bonded magnet instead of a sintered magnet in the applications where sintered magnets are normally used, further improvement is required for such a bonded magnet.
The present invention is devised in light of the above. It is one object of the present invention to provide a bonded magnet which has an increased magnetic flux density while having the bonded magnet structure.